Redox history of the Three Gorges region during the Ediacaran and Early Cambrian as indicated by the Fe isotope

Abstract

The Ediacaran–Cambrian transition is characterized by numerous events such as the emergence of large multi-cellular metazoans and surface environmental disturbances. Based on geological evidence, it has been proposed that this transition coincided with the increase in the atmospheric oxygen level that was key to the evolution of life. Even though ancient redox conditions can be inferred from the composition of sedimentary iron mineral species, this method is not necessarily applicable to all rocks. In the Earth system, the cycling of iron is of considerable interest owing to its sensitivity to redox conditions. Information regarding the paleo-oceanic iron cycle is revealed in the iron isotopic composition of iron-bearing minerals. Unfortunately, only limited iron isotopic data exists for Ediacaran- to Cambrian-period oceans. To circumvent this deficiency, we drilled a fossiliferous Ediacaran to Early Cambrian sedimentary succession in the Three Gorges region, South China. We analyzed the iron isotope ratios (δ56/54Fe) of pyrite grains in the drill cores using laser ablation multi collector inductively coupled plasma mass spectrometry. The results demonstrate large variations in δ56/54Fe, from −1.6 to 1.6‰ and positive iron isotope ratios are observed in many successions. The presence of positive δ56/54Fe in pyrite indicates that the ferrous iron in the seawater was partially oxidized, suggesting that seawater at Three Gorges was ferruginous during the Ediacaran and Early Cambrian periods. However, aggregated pyrite grains in organic carbon-rich black shales at Member 4 of the Doushantuo Formation and the base of the Shuijingtuo Formation yield near-zero δ56/54Fe values; this suggests that the ocean was transiently dominated by sulfidic conditions during these periods. Notably negative δ56/54Fe values, lower than −1‰ can be interpreted as a signature of DIR. The DIR also might contribute in part to the re-mineralization of organic matter during the largest negative carbon isotope anomaly in the Ediacaran.